Low energy, high recovery, rapid cycle kinetic PSA for biogas

ABSTRACT

Disclosed herein are embodiments of a rotary gas separation device, such as a rotary pressure swing adsorption device. The rotary pressure swing device can include, for example, a rotor with a plurality of adsorber elements, a stator with a plurality of conduits, and a rotary valve comprising a seal assembly positioned between the rotor and the stator.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of the earlier filing date of U.S.Provisional Application No. 61/350,355, filed Jun. 1, 2010, which isincorporated herein by reference.

FIELD

This disclosure concerns gas separation devices, such as rotary pressureswing adsorption devices that can be used to separate methane from afeed gas, such as biogas.

SUMMARY

Certain disclosed embodiments of the present invention concern a gasseparation method for separating carbon dioxide from a gas mixturecomprising methane and carbon dioxide using a gas separation device. Oneexemplary disclosed device comprises a rapid cycle kinetic pressureswing adsorption apparatus utilizing an adsorbent structure having akinetic adsorbent comprising a molecular sieve with an average pore sizeof less than about 3.9 Angstroms, and in certain embodiments an averagepore size greater than about 3.7 Angstroms. Exemplary molecular sievesinclude silica alumina phosphate, such as SAPO 34. The crystal sizedistribution of the SAPO 34 typically is between 10 and 80 microns, andmay have an average crystal size of about 40 microns. In certainembodiments, the adsorbent structure is characterized by a macroporositybetween about 4% and about 18%, and a macropore size between about 0.001and about 2 microns. The adsorbent structure may be a parallel passagestructured adsorbent comprising a laminated adsorbent sheet. The gasseparation device may be a rotary rapid cycle kinetic pressure swingadsorption apparatus.

Embodiments of a pressure swing process cycle for separating carbondioxide from a gas mixture comprising methane and carbon dioxide alsoare described. For example, an exemplary process may comprise providinga gas separation device having an adsorbent structure, introducing thegas mixture at an elevated pressure to the adsorbent structure wherebycarbon dioxide is preferentially kinetically adsorbed by the adsorbentto produce a methane-rich product gas, delivering the methane-richproduct gas from the apparatus, reducing the pressure in the apparatuswhereby adsorbed carbon dioxide is desorbed from the adsorbent toproduce a carbon-dioxide-rich exhaust gas, and delivering thecarbon-dioxide-rich exhaust gas from the apparatus. The process stepsmay be repeated in a rapid cycle, such as a rapid cycle with cyclespeeds of from about 2 cycles per minute (CPM) to at least about 30 CPM,with certain embodiments using a speed of 13 CPM. For certainembodiments, the product producing step time is determined to be longerthan 50% of the time required for CO₂ to reach total equilibrium onadsorption sites. For certain embodiments, the product producing steptime is determined to be shorter than 10% of the time required for CH₄to reach total equilibrium on adsorption sites. The evacuation step timemay be determined to be longer than 90% of the time required for CO₂ toreach total equilibrium on adsorption sites. And, the ratio of the timespent in the production step to that spent in the evacuation step may beless than 0.15. For certain embodiments, the ratio of the time spent ina single equalization step to that spent in the production step is lessthan 0.25. The co-current depressurization steps may be applied togenerate Reflux 1 recycle streams. Co-current depressurization steps maybe vacuum assisted to generate Reflux 2 recycle streams. And, a portionof exhaust gas may be used as a co-current purge stream to recoverremaining methane into reflux or recycle streams.

Embodiments of a method for making an adsorbent structure, such as alaminated sheet, for a pressure swing adsorption apparatus also aredisclosed. The adsorbent structure may comprise an adsorbent sensitiveto sudden increases or decreases in water content. An exemplaryembodiment of the method comprises preparing an aqueous slurry,comprising up to 50% water, and a sensitive adsorbent. A substrate, suchas a sheet, is coated with the aqueous slurry. The coated substrate isdried to produce the adsorbent structure.

Drying may comprise plural drying steps, such as first, second, andthird drying stages in sequence. For these embodiments, the rate ofdrying in the second drying stage typically is slower than that in thefirst drying stage and the rate of drying in the first drying stagetypically is slower than that in the third drying stage. The firstdrying stage may involve: drying at a drying rate removing less than orequal to 30% weight of water per minute; drying at less than or equal to40° C. on exit of dryer; and/or may comprise drying such that 25 to 30%of the water in the coated substrate is removed before the dryingprocess is discontinued. For certain embodiments, the second dryingstage may comprise: drying at a rate removing less than or equal to 5%weight of water per day; drying at ambient temperature; and/or dryingsuch that 5 to 10% of the water in the coated substrate is removedbefore drying is discontinued. For certain embodiments, the third dryingstage may comprise: drying at a drying rate removing greater than orequal to 10% weight of water per minute; drying at a temperature aboveabout 130° C.; and/or drying such that 5 to 10% of the water in thecoated substrate is removed before drying is discontinued. For certainembodiments, up to 10% of the water in the coated substrate remains inthe adsorbent in the adsorbent structure after drying.

The method also may include exposing sensitive adsorbent to water vapor,such as ambient humid air, before preparing the slurry. This processslowly adds water to the adsorbent before the slurry is prepared. Theexposing step typically occurs over a period of from about 4 to about 7days. And from about 20% to about 30% by weight water may be adsorbed bythe sensitive adsorbent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one pressure swing embodiment of thedisclosed rotary gas separation device.

FIG. 2 is a plan view of the bottom of the rotary pressure swing deviceembodiment shown in FIG. 1.

FIG. 3 is a plan view of the top of the rotary pressure swing deviceembodiment shown in FIG. 1.

FIG. 4 is a cross-sectional view of the rotary pressure swing deviceembodiment shown in FIG. 1 taken along the line 4-4 in FIG. 3.

FIG. 5 is a perspective view of the rotor of the rotary pressure swingdevice embodiment shown in FIG. 1.

FIG. 6 is a plan view of the top of the rotor of the rotary pressureswing device embodiment shown in FIG. 1 with a portion cut away to showa cross-section taken through the adsorber elements.

FIG. 7 is a cross-sectional view of the rotor of the rotary pressureswing device embodiment shown in FIG. 1 taken along the line 7-7 in FIG.6.

FIG. 8 is a detailed view of the flow passage and seal shown in FIG. 7.

FIG. 9 is a schematic cross sectional view showing the pressures andforces acting on the seal assembly shown in FIG. 8 during operation ofthe rotary pressure swing device.

FIG. 10 is a plan view of the top end of one embodiment of an adsorberelement suitable for use with embodiments of the disclosed rotarypressure swing device.

FIG. 11 is a cross sectional view of the adsorber element embodimentshown in FIG. 10 taken along the line 11-11.

FIG. 12 is a plan view of a spacer cross support structure betweenlaminate structures in the adsorber element embodiment of FIG. 11.

FIG. 13 is a close-up partial cross sectional view of the adsorberelement embodiment shown in FIG. 10 taken along the line 13-13.

FIG. 14 is a schematic that represents the adsorption process concept.

FIG. 15 is a table of the process steps, including relative timing andprocess pressures.

FIG. 16 is a chart of the process steps, showing the rapid kineticpressure swing adsorption process steps and flows, with 28 beds, 5equalization, two refluxes, and one heavy rinse.

DETAILED DESCRIPTION

Unless otherwise explained, all technical and scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which this disclosure pertains. The singular terms“a,” “an,” and “the” include plural referents unless the context clearlyindicates otherwise. Similarly, the word “or” is intended to include“and” unless the context clearly indicates otherwise. The term“includes” means “comprises.” All patents and publications mentionedherein are incorporated by reference in their entirety, unless otherwiseindicated. In case of conflict as to the meaning of a term or phrase,the present specification, including explanations of terms, willcontrol. Directional terms, such as “upper,” “lower,” “top,” “bottom,”“front,” “back,” “vertical,” and “horizontal,” are used herein toexpress and clarify the relationship between various elements. It shouldbe understood that such terms do not denote absolute orientation (e.g.,a “vertical” component can become horizontal by rotating the device).The materials, methods, and examples recited herein are illustrativeonly and not intended to be limiting.

Exemplary Device Configuration

FIGS. 1-13 show various views of one embodiment of the disclosed rotarypressure swing device. The illustrated rotary pressure swing device 100includes a rotor assembly 102 positioned between an upper statorassembly 104 and a lower stator assembly 106. As best seen in FIG. 4, amotor (not shown), can be used to rotate an axle 114 connected to arotor 116 within the rotor assembly 102. Below the rotor 116, a bottombearing 117 supports the mass of the rotor 116 and permits rotation ofthe rotor 116. Above the rotor, a top bearing 115 aligns the axis of therotor 116 with the axis of the rotary pressure swing device 100. Therotor 116 is contained within a rotor housing 119.

The rotor 116 is useful for rotating adsorber elements 122. Rotation ofthe rotor 116 cycles fluid flow through each adsorber element 122 as anadsorber element is rotated to receive fluid flow through a fluid port.While various processes can be implemented using disclosed embodimentsof rotary pressure swing devices, for one exemplary process eachcomplete cycle includes a high-pressure adsorption stage and alower-pressure exhaust stage. In addition, some embodiments include oneor more reflux stages between the high-pressure adsorption stage and thelower-pressure exhaust stage. Embodiments also can include one or morefeed pressurization stages prior to the high-pressure adsorption stageand/or one or more product purge stages prior to the low pressureexhaust stage. Additional details regarding the disclosed rotarypressure swing processes are provided later in this disclosure under thesubheading “Exemplary Process Specifications.” Pressure transducers (notshown) can be included at one or both ends of representative adsorberelements 122 to generate data for monitoring and calibrating the processcycles. Signals from the pressure transducers can be fed to acontroller.

The illustrated embodiments of rotary pressure swing device 100 areconfigured to perform one complete PSA cycle for each rotation of therotor 116. Other embodiments can be configured to perform a greaternumber of PSA cycles per rotation, such as two, three or four. Includingmore than one PSA cycle per rotation can be advantageous for efficientproduct production and for other process considerations, such as todistribute pressure loads more evenly around the circumference of thedevice and to reduce the overall rotation speed of the device for agiven PSA cycle speed.

As best seen in FIG. 2, the feed mixture flows into feed port 126. Thelower stator assembly 106 also includes one lower reflux port 132.

The illustrated embodiments of the PSA device 100 also typically includeexhaust ports 140. There may be one, two or more exhaust ports. Multipleexhaust ports can be used to reduce the pressure drop through theexhaust ports by dividing the flow and reducing the gas velocity.

The configuration of the upper stator assembly 104 is best seen in FIG.3. Product exits the rotary pressure swing device 100 via a product port144. In operation, a product pressurization conduit (not shown) conveysfluid from the product port 144 to the product pressurization port 148.Flow through the product pressurization conduit (not shown) can becontrolled by a product pressurization conduit valve (not shown). Inoperation, a product purge conduit (not shown) conveys fluid from theproduct port 144 to the product purge port 152, to deliver purge fluid,which can be product fluid, to the adsorber elements 122. Flow throughthe product purge conduit (not shown) can be controlled by a productpurge conduit valve (not shown). The upper stator assembly 104 alsoincludes upper reflux #1 port 156 and upper reflux #2 port 158.

As best seen in FIG. 4, the upper stator plate 168 and the lower statorplate 170 are effectively coupled to the rotor housing 119. Attached tothe upper stator plate 168 is an upper seal counter-face 169, andattached to the lower stator plate 170 is a lower seal counter-face 171.Reflux channels 150 in the upper stator plate 168 and upper sealcounter-face 169 can be used to convey reflux flows from one stage inthe process to another, according to the required process. Product wearplate channels 151 convey fluid from the rotor 116 to ports 152, 148,144, 156, and 158 in the product stator assembly 104, according to therequired separation process. Feed wear plate channels 111 convey fluidfrom the rotor 116 to ports 126, 140, and 132 in the feed statorassembly 106, according to the required separation process. Additionaldetails regarding the disclosed rotary pressure swing processes areprovided later in this disclosure under the subheading “ExemplaryProcess Specifications.”

The upper seal assembly 179 is positioned between the upper sealcounter-face 169 and the upper rotor end plate 172. Similarly, a lowerseal assembly 180 is positioned between the lower seal counter-face 171and the lower rotor end plate 174.

FIG. 5 is an isometric view of the rotor 116. The rotor 116 includes anupper rotor end plate 172 and a lower rotor end plate 174, both centeredand fixed rotationally to axle 114. A ring of inner tie rods 184 and aring of outer tie rods 186 secure the upper rotor endplate 172 to thelower rotor endplate 174. The inner and outer tie rods 184, 186 aresecured at their bottom end by threading them into a partially threadedhole in the lower rotor endplate 174. The inner and outer tie rods 184,186 are secured at their top end with nuts 188 pressing against theouter surfaces of the upper rotor end plate 172. As best seen in FIG. 7,once secured, the top and bottom ends of the bed tube 228 press againstthe inner surfaces of the upper and lower rotor end plates 172, 174,respectively. This secures the adsorber elements 122 in the rotor 116and prevents them from moving. Also shown in FIG. 5 is the product sealassembly 179. Additional details regarding the disclosed rotary pressureswing seals are provided later in this disclosure under the subheading“Exemplary Seals.”

FIG. 6 includes a partial cut-away showing the arrangement of theadsorber elements 122 in the rotor 116 for the illustrated embodiment ofPSA device 100. Packing efficiency may be improved by using a greaternumber of relatively small adsorber elements 122 in multiple rows,rather than a smaller number of relatively large adsorber elementsarranged in a single row. This positions the adsorber elements 122 in anested, interlocking, radial pattern. In the illustrated embodiment, therotary pressure swing device 100 includes 28 adsorber elements 122 intwo concentric rows of 14 adsorber elements 122. Of course, differentembodiments can have different numbers of adsorber elements in differentspatial arrangements. Many design features of the rotary pressure swingdevice 100 are considered when selecting the arrangement of theadsorbent elements 122. Such features include the overall pitch circlediameter of the upper and lower seal assemblies 179, 180, the rotationspeed, the gas flow efficiency, and dead gas volume. For illustration,inner adsorber elements 122A, 122B, and outer adsorber elements 122C,122D are shown. A portion of adsorber element 122A in the inner pair ispositioned between adsorber elements 122C and 122D in the outer pair anda portion of adsorber element 122D in the outer pair is positionedbetween adsorber elements 122A, 122B in the inner pair. This nestedconfiguration allows for closer packing of adsorber elements 122 thanwould be possible if the pairs of adsorber elements were arranged in auniform ring and allows for uniform fluid flow to all of the adsorberelements 122. An even number of adsorber elements 122 facilitates thisarrangement.

FIG. 7 shows a cross sectional view of rotor 116. Fluid from the upperends of adsorber elements 122 is conveyed via angled drillings 181 andseal connectors 191 to the upper rotor apertures 176. These upper rotorapertures 176 communicate with product wear plate channels 151.

Fluid from the lower ends of adsorber elements 122 is conveyed viaangled drillings 182 and seal connectors 192 to the lower rotorapertures 178. These lower rotor apertures 178 communicate with feedwear plate channels 111.

Exemplary Seals

FIGS. 8-9 illustrate an embodiment of the upper seal assembly 179 shownin FIG. 7. In the rotary pressure swing device 100, the upper sealassembly 179 is attached to the upper rotor endplate 172, and rotateswith the rotor 116 during operation. The lower seal assembly 180 (FIG.7) is comparable in structure to the upper seal assembly 179, so onlythe upper seal assembly is illustrated and described in detail. Bothassemblies however serve as part of rotary distributor valves todistribute gases to and from the various adsorber elements atappropriate times during the pressure swing cycle, and yet also providean adequate seal against leakage of these gases. As the rotor rotatesbetween the wear plates (seal counterfaces), valve action is provided asapertures in the wear plates and seal assemblies come into and then outof alignment.

As shown in FIG. 8, the upper seal assembly 179 is a “floating seal,” asthe upper seal assembly 179 is free to move vertically relative to therest of the rotor 116. Upper seal assembly 179 is composed of a sealbacker 306 and a wear portion 308 with a sealing face 309. Slidingcontact with the upper seal counter-face 169 occurs at the sealing face309.

It is advantageous to provide sufficient closing force at the sealingface 309 to prevent substantial material loss, while still minimizingfriction that can increase torque and cause excessive wear on thesliding parts. The illustrated upper seal assembly 179 includes severalfeatures that facilitate achieving an appropriate closing force. FIG. 9is a schematic, cross sectional view showing the pressures and forcesacting on different surfaces of the upper seal assembly 179 duringoperation of the rotary pressure swing device 100. Arrows 326 (onenumbered in FIG. 9) indicate the “process pressure,” which is thepressure of the process gas in the adsorber elements 122. The pressureof the process gas varies significantly at different stages of the PSAcycle, so the process pressure indicated by arrows 326 also varies.Arrows 328 (one numbered in FIG. 9) indicate the force on the sealbacker 306 exerted by an inner row of springs (not shown), and an outerrow of springs (not shown), which are placed between the seal backer 306and product rotor endplate 172. Arrows 330 indicate the “openingpressure,” which is the pressure against the sealing face 309 and upperseal counter-face 169 that is exerted by leakage of the process gas. Inthe illustrated example, the opening pressure diminishes with furtherdistance from the rotor aperture 176, as indicated by the relativelengths of the arrows 330.

Directly adjacent to the rotor aperture 176, the opening pressure issubstantially equal to the process pressure. At the far edge of thesealing face 309, the opening pressure is substantially equal to thepressure inside the rotor housing 119. The opening pressure across thesealing face 309 from the rotor aperture 176 to the far edge candecrease according to a variety of profiles. In some implementations,the profile is linear, as shown by line 332 in FIG. 9. The profile alsocan be convex or concave. The total opening pressure across the sealingface 309 from the rotor aperture 176 to the far edge can be calculatedusing modeling.

The variables that can be adjusted to achieve the target net closingforce include the spring force and the area on the bottom surface of theseal backer 306 that is exposed to the process pressure 326.

A variety of materials can be used in embodiments of the disclosed uppersealing assembly 179. Material properties such as coefficient of theanalexpansion, stiffness (modulus and thickness), and thermal conductivityare useful to consider. Overall stiffness of the backer 306 typicallypromotes stable operation over time. Some compliance, however, isdesirable to accommodate imperfections in the flatness of the upper sealcounter-face 169.

In some embodiments, the wear portion 308 is made of a polymericmaterial or materials, such as filled polytetrafluoroethylene, and theremainder of the seal backer 306 is made of a metal, metal alloy, orcombinations thereof, such as carbon steel. The wear portion 308 can becoupled to the remainder of the seal backer 306 by any suitable means,such as by using epoxy.

While the PSA device is operating, hot flowing gases and friction heatthe various components making up the rotary face seal. Temperaturegradients can form and the components will expand and can distort. Inparticular, if the thermal expansion coefficient of the wear portion 308is significantly greater than that of seal backer 306, the former wantsto expand more than the latter. But, because they are bonded together,the wear portion 308 then can get distorted. Both a coning (developing aconvex shape over the diameter of the seal ring) and a crowning(developing a convex shape over the width of the seal face) of the sealassembly 179 and the seal face 309 can occur.

We have discovered that this crowning can be reduced substantially andthus the seal can be stabilized against temperature variations byincorporating certain anti-crowning features into the seal ring. Theanti-crowning relief slot feature 165 (seen best in FIGS. 5 and 6) cancomprise a blind relief slot in the middle of the seal face such thatthe slot does not go all the way through the seal face to the backingring. The seal ring may comprise a plurality of such blind relief slots.

Exemplary Adsorber Elements

FIGS. 10 and 11 illustrate one embodiment of an adsorber elementsuitable for use with embodiments of the disclosed rotary pressure swingdevice. As shown in FIG. 11, the adsorber element 122 includes pluraladsorbent beds. For example, one embodiment includes a first radialadsorber 202, a second radial adsorber 204, a third radial adsorber 206,and a fourth radial adsorber 208. The first, second, third, and fourthradial adsorbers 202, 204, 206, 208 are laminate sheets concentricallywound about first, second, third, and fourth mandrels 210, 212, 214,216, respectively. Spacers can be used to space adsorbers. For example,three epoxy spacer crosses 218 are positioned between the first andsecond radial adsorbers 202, 204, the second and third radial adsorbers204, 206, and the third and fourth radial adsorbers 206, 208,respectively. The epoxy spacer crosses 218 help prevent axialdisplacement of the laminate sheets as a result of pressurefluctuations. Adsorber element 122 includes a feed bed connector 220adjacent the first radial adsorber 202, and a product bed connector 222adjacent to the fourth radial adsorber 208.

A button head screw 224 attaches the feed bed connector 220 to the firstmandrel 210. A second button head screw 224 attaches the product bedconnector 222 to the fourth mandrel 216. Set screws 226 attach thefirst, second, third, and fourth mandrels 210, 212, 214, 216 to eachother. Housing 228 extends around the internal components of theadsorber element 122 between the feed bed connector 220 and the productbed connector 222. Two housing o-rings 230 a and 230 b are positionedaround the feed bed connector 220, and also around the product bedconnector 222, to facilitate fluidly sealing housing 228 to the feed bedconnector 220 and the product bed connector 222. O-ring 230 a is asacrificial o-ring, used to stop sealant and protect o-ring 230 b so itmight effectively seal gas. Two connector o-rings 232 are positionedaround the feed bed connector 220 and the product bed connector 222,respectively, to facilitate fluidly sealing the feed bed connector andthe product bed connector to other portions of the overall rotarypressure swing device 100. For example, in the rotary pressure swingdevice 100, the feed bed connector 220 and the product bed connector 222can be sealed into a product connector cavity 142 (FIG. 7) and a feedconnector cavity 143 (FIG. 7), respectively.

As shown in FIG. 11, embodiments of adsorber elements (e.g., theillustrated adsorber element 112) suitable for use with embodiments ofthe disclosed rotary pressure swing device 100 typically include two ormore sections, and each may contain a different adsorbent material or adifferent combination of adsorbent materials. As used herein, the term“adsorbent material” refers to particular types of adsorbent materialand to particular combinations of types of adsorbent material.

In some embodiments, support structures are included to reducedisplacement. Examples of support structures include the epoxy spacercrosses 218 illustrated best in FIGS. 11 and 12. Such support structurescan be positioned between multiple, separate laminate structures or atone or both ends of individual laminate structures. Support structurestypically include one or more elongated portions 218 a (e.g., spokes)extending across the face of the laminate structures substantiallyperpendicular to the direction of possible displacement, such as theaxial direction of spirally-wound laminate structures or the directionof fluid flow through the adsorber element. These elongated portions canbe rigid or flexible and can be sized so as to minimize the disruptionof fluid flow through the overall adsorber element.

The support structures can be bonded to none, one, or both of theadjacent laminate structures. In some embodiments, such as the adsorberelement 122 illustrated in FIGS. 10-12, the support structures comprisean adhesive material, such as an epoxy, capable of bonding to one orboth of the adjacent laminate structures. The adhesive material can beactivated, for example, by heating the overall adsorber element, such aswhile activating the adsorbent materials. The support structures alsomay be solely an adhesive material, such as an epoxy. Adhesive materialsoften can be applied expediently to the faces of laminate structures,such as by deposition in gel form. These materials then can be allowedto set to become strong enough to resist laminate displacement.

A preferred embodiment comprises spacer cross support structures 218that are formed by applying liquid epoxy resin to the ends of thelaminate structures in the shape of a cross. A template may be employedto assist in applying beads of epoxy in order to form an appropriatecross shape 218 a. The epoxy resin can thus work its way to somesignificant extent into the ends of the wound laminate structure and,after curing, thereby provide useful support against collapse of thewindings at the ends. This can be a significant improvement over simplyusing preformed supports as these do not provide support between theactual winds in the laminate structure itself.

Mandrels, such as mandrels 210, 212, 214, 216 of the adsorber element122, can be attached to inner surfaces of laminate structures andconstrained from axial movement. This helps to hold the laminatestructures in place. The attachment can result by any suitable means,such as by adhesive bonding. In addition, the adsorber element housing228 can be used to help hold the laminate structures in a desiredposition. For example, a laminate structure can be placed into a housing228, and then a substantially uniform concentric compressive forceapplied to the housing to deform it about the laminate structure toreduce or substantially prevent axial movement of the windings. Tofurther reduce or substantially prevent movement of the laminatestructures and to reduce or substantially prevent gas flow out of thehousing, a bead of material, such as a ceramic material, can be placedabout the inner circumference of the shell. In the adsorber element 122,a filter can be positioned to contact this bead. Other methods offastening also can be used, such as a shoulder built into the shell.Sealants suitable for attaching laminate structures to shells includeepoxy resins, such as LOCTITE® HYSOL® E-120 HP epoxy available fromHenkel Corporation (Rocky Hill, Conn.). For high temperatureapplications (e.g., greater than about 130° C.), suitable sealantsinclude PYRO-PUTTY® 653 available from Aremco Products, Inc. (ValleyCottage, N.Y.).

Between the laminate structures 202, 204, 206, 208 and the adsorberelement housing 228 (FIG. 13) an impermeable barrier 224 can be used toprevent sealant from permeating the laminate structure 208. Thisimpermeable barrier can be a polymer tape, or polymer sleeve, such asthe shrink-wrap used to protect electrical cables. Sealant can beinjected via the seal injection port 240.

Mandrels, housings and other components (e.g., support structures) ofdisclosed adsorber elements can be made from a variety of materials,such as metals and metal alloys (e.g. stainless steel), ceramics and/orpolymeric materials. In some embodiments, the shell and/or the mandrelhave a thermal conductivity at room temperature of from about 10 toabout 1000 W/(m·° C.), such as from about 20 to about 1000 W/(m·° C.) orfrom about 50 to about 1000 W/(m·° C.). The adsorbent materials used toform laminate structures can be activated, if necessary, subsequent toinsertion into the housing. In such cases, both the housing and themandrel are sufficiently robust to withstand the adsorbent-activationtemperature, such as temperatures of about 250° C. and greater. Foradsorbent materials that can be activated at lower temperatures, thematerial used to form the shell and mandrel can be other than metals,metal alloys, ceramics, etc.

Exemplary Adsorbent and Process Specifications

Molecular sieves are materials with precise pores in the Angstrom sizerange. Among different molecular sieves, Zeolitic Molecular Sieves andCarbon Molecular Sieves are used widely in gas separation processes. Themost common examples used in the gas separation industry are nitrogenseparation from air or carbon dioxide separation from natural gas.

Our new separation process utilizes a newly designed rapid cycle kineticpressure swing adsorption (RkPSA) process cycle, which is tailored tomatch the properties of suitable adsorbent crystals, our adsorbentsheets, and our structured adsorbent. Our new process has beendemonstrated to enable us to achieve methane product purities of97%-99%, at high recoveries of 95%-99%, with minimum energy consumptionof 0.2-0.3 kW per normal cubic meter per hour of feed gas.

Adsorbent:

New molecular sieves were developed to create an improved gas separationprocess. A novel process to purify methane from bio gas sources has beendeveloped. The new process may utilize a selected adsorbent powder fromthe silica alumina phosphate family called SAPO 34. SAPO 34 is from thefamily of Chabazites having a chemical formula of |Ca2+6(H2O)40|[Al12Si24O72]-CHA.

SAPO 34 molecular sieve has an 8 ring member pore with reported poresizes in the range from 3.7-4.2° A. The Database of Zeolite Structuresmentions an average pore size of 3.8 Degree Angstrom (° A). In thisinvention a SAPO 34 adsorbent having less than 3.9° A pore size is usedto separate CO₂ molecules (with kinetic diameter of 3.4° A) from CH₄molecules (with kinetic diameter 3.9° A). For this inventive process thespecific adsorbent crystals have three important characteristics:

-   -   A. A molecular sieve pore size of less than 3.9° A, e.g. 3.8° A.        Accurate pore size control is important in maintaining high        kinetic selectivity by providing for fast diffusivity for CO₂        and slow diffusivity for CH₄. In this application to separate        CO₂ (3.4° A) from CH₄ (3.9° A), the specific adsorbent crystals        have to maintain pore sizes between these values.    -   B. A unimodal log-normal crystal size distribution of 10 to 80        micron is important, e.g. adsorbent having an average crystal        size of 40 micron. Maintaining a unimodal and tight crystal size        distribution is important in maintaining sharp kinetic        selectivity. The mass transfer rate of a gas is a function of        the diffusion path; hence the process is slower for large        crystals than small crystals.    -   C. An equilibrium capacity of at least 50 cc of CO₂ per gram of        adsorbent under standard conditions is typically desired to        reach industrial performance targets for this separation        application. It is expected that CO₂ equilibrium capacities of        70 cc/g can be achieved. Preferably up to 100 cc/g of CO₂        capacity can be achieved. The equilibrium capacity of SAPO 34        adsorbent for CH₄ is reported to be 10 cc/g. The CH₄ equilibrium        capacity should be as low as possible, preferably as low as 5        cc/g, to maximize the equilibrium selectivity of CO₂/CH₄. The        CO₂/CH₄ equilibrium selectivity of the specific adsorbent        crystals under standard conditions can be as high as 20 cc/g.        Slurry:

We have discovered that the specific adsorbent crystals are unexpectedlyvery sensitive to the rate at which the moisture content is varied. Forinstance, we have noticed that the adsorbent loses a significant portionof its adsorption capacity when it suddenly comes into contact withliquid water (i.e. a sudden increase in water content) during a typicalslurry preparation process (such slurries being typically used incoating process to make adsorbent sheets for adsorbent beds). This isbelieved to happen because the hydrogen bonds in the specific adsorbentcrystals are hypersensitive and not very strong and can break off or beexchanged in the slurry solution. To minimize this capacity loss, wehave developed a slow rehydration process in which we expose previouslydry adsorbent to air humidity for 4-7 days. In this process moistureenters the pores in a more controlled fashion with minimum disturbanceof the specific adsorbent crystals structure. After this slowrehydration, about 5-10 wt % water was found to be adsorbed in thecrystals and the adsorbent does not lose capacity when subsequently putin contact with liquid water.

Drying after Coating:

A typical slurry for coating the adsorbent contains 50 wt % water. Atthe end of the drying process, the coated adsorbent contains 5-10 wt %water only. A conventional drying process takes only a few minutes forother adsorbents. In like manner to the above sudden increase inmoisture content, the specific adsorbent crystals are also verysensitive to sudden decreases in moisture content. We have found thatthe adsorbent loses up to 90% of its capacity if the coated slurry isdried within in several minutes. Thus a three stage drying process hasbeen developed to preserve the capacity of the specific adsorbentcrystals during the preparation of coated adsorbent sheets. A gentle IRdrying, followed by slow drying in ambient, and completed by hightemperature IR treatment is explained below:

-   -   a. The adsorbent sheet is dried so as to lose 25-30% of its        water by gentle IR heating (500 watts/sqft laminate) while        maintaining ˜40° C. coating line temperature.    -   b. Then the partially dried adsorbent sheet is stored for 5-10        days while it slowly loses another 5-10% of its water to the        ambient atmosphere. Once the bulk of the water has evaporated        from the adsorbent sheets, the sheets have to get dried further        to improve the strength.    -   c. IR Treatment: The adsorbent sheets prepared using the        specific adsorbent crystals could have strength and uniformity        issues if not treated by a last faster drying stage (e.g. by        IR). The otherwise low strength obtained is a direct result of        less uniformity in the coating caused by migration of binder and        fine particles to the surface of adsorbent sheets during drying        stages. This was confirmed by observing a hard thin film of        coating on the adsorbent sheet surface. Such a non-uniform        porous media could become an obstacle for achieving a desired        kinetic separation driving force by interfering with gas        diffusion rates. In order to improve the strength of the        adsorbent sheets, a rapid drying process was employed so the        remaining water in the adsorbent sheets was removed without        allowing binder migration. In this process the adsorbent sheets        were passed through a high power IR heater zone (5000 watts/sqft        laminate) reaching as high as 250° C. The high power IR        treatment removes an additional 5-10% of the water without        changing the particle size distribution of the coating. At the        end of this stage the adsorbent sheets contain 5-10% moisture.        The result of this stage is an adsorbent sheet with acceptable        strength and proper moisture content.        Adsorbent Sheet:

The structured adsorbent sheets were formed with thicknesses rangingfrom 0.009″ to 0.015″. The adsorbent sheets formulation was optimized toproduce low macroporosity in a range of 4% to 18% with macropore size of0.001 to 2 micron. It is important to try and maintain such a lowmacroporosity in adsorbent sheets. Any macroporous media between thecrystals can act as void space trapping CH₄ molecules and hence wastingthem to the exhaust stream during a counter current blow down step. Ahomogenous adsorbent sheet with low macroporosity can readily beachieved by using the proper crystal size distribution mentionedpreviously and by using the aforementioned drying procedure.

RkPSA Cycle:

The new rapid cycle kinetic pressure swing adsorption separation worksbased on the differences between the kinetic size of molecules and theeffective size of the adsorbent pores, hence is called kinetic pressureswing adsorption or kPSA. This process is described below in text, andin FIGS. 15-16.

A brief description of the RkPSA process is that when the bed pressureincreases, CO₂ molecules, which are smaller compared to the adsorbentpore size, diffuse rapidly through the adsorbent pores and into theadsorption sites. However, the CH₄ molecules, which are larger comparedto the adsorbent pore size, cannot diffuse into the adsorbent pores in ashort production step. Hence CH₄ molecules occupy mainly the gas phasein the bed void space. The bed void space generally consists of the gaschannels and macroporous media of adsorbent sheets. The result is anincreased concentration of methane in the gas phase which exits the bedfrom the product port as purified methane during the production step.Hence once the bed pressure increases to its maximum level, the productvalve opens to withdraw the purified methane from the bed.

The product step continues until the adsorbent pores are mainly occupiedby CO₂ molecules. At this point the product valve shuts off to preventlow purity methane breakthrough. The product step should be shorter thanthe time that CH₄ needs to diffuse into the crystals. This helpsmaintaining high CH₄ concentration in equalization steps and high CO₂concentration in exhaust steps.

At the end of the production step there is still a significant amount ofCH₄ in the bed at high pressure to be recovered. Recovery of the leftover gas from the adsorbent vessels is done by using several recyclestreams inside and outside of the RkPSA machine.

Internal recycle streams are utilized for equalization steps. The timethat the adsorbent beds spend in equalization steps should be shorterthan the time required for CO₂ to diffuse out of the adsorbent crystals.In other words, the rate of equalization is faster than the rate ofdiffusion of CO₂ out of the adsorbent. This helps maintain high CH₄concentration in the equalization steps and high CO₂ concentration inthe exhaust steps.

In a preferred embodiment, the external recycle streams are utilized fortwo external refluxes. The first reflux stream recovers CH₄ gas from bedvoids until the bed pressure drops close to atmospheric pressure. Thesecond reflux stream recovers the additional CH₄ from the bed voidspaces under vacuum. To recover the remaining CH₄ trapped in the bedvoid space a heavy rinse stream pushes high CO₂ content of exhaust gasto the bottom of the bed. The CO₂ moves up in the bed while diffusinginto all bed void spaces and adsorbent sheet macropores. This actionpushes the CH₄ gas out of the voids and macropores and towards theproduct end of the bed.

Once most of the CH₄ is recovered from a bed, the exhaust valve opens todepressurize the bed, thereby helping CO₂ molecules exit the adsorbentpores and eventually the adsorbent beds. The exhaust step has to be longenough to remove all CO₂ from the adsorbent sites. It should also belong enough to help CH₄ diffuse out of the adsorbent.

This process is repeated on all beds sequentially to generate steadystate product and exhaust flows. The frequency of the cycle is 30 CPM,but it could be anywhere from 1 to 100 CPM.

In view of the many possible embodiments to which the principles of thedisclosed invention may be applied, it should be recognized that theillustrated embodiments are only preferred examples of the invention andshould not be taken as limiting the scope of the invention. Rather, thescope of the invention is defined by the following claims. We thereforeclaim as our invention all that comes within the scope and spirit ofthese claims.

We claim:
 1. A method of making an adsorbent structure for a pressureswing adsorption apparatus wherein the adsorbent structure comprises anadsorbent sensitive to sudden increases or decreases in water content,the method comprising: preparing a slurry of the sensitive adsorbent andwater; coating a substrate with the aqueous slurry; and drying thecoated substrate to produce the adsorbent structure wherein the dryingcomprises first, second, and third drying stages in sequence wherein therate of drying in the second drying stage is slower than that in thefirst drying stage and the rate of drying in the first drying stage isslower than that in the third drying stage.
 2. The method of claim 1wherein the slurry comprises up to 50% by weight of water.
 3. The methodof claim 1 wherein the first drying stage comprises using a drying ratethat removes less than or equal to 30% weight of water per minute. 4.The method of claim 1 wherein the first drying stage comprises drying atless than or equal to 40° C. on exit of a dryer.
 5. The method of claim1 wherein the first drying stage comprises removing 25 to 30% of thewater in the coated substrate before drying is discontinued.
 6. Themethod of claim 1 wherein the second drying stage comprises using adrying rate that removes less than or equal to 5% weight of water perday.
 7. The method of claim 1 wherein the second drying stage comprisesdrying at ambient temperature.
 8. The method of claim 1 wherein thesecond drying stage comprises removing 5 to 10% of the water in thecoated substrate before drying is discontinued.
 9. The method of claim 1wherein the third drying stage comprises using a drying rate thatremoves greater than or equal to 10% weight of water per minute.
 10. Themethod of claim 1 wherein the third drying stage comprises drying at atemperature above 130° C.
 11. The method of claim 1 wherein the thirddrying stage comprises removing 5 to 10% of the water in the coatedsubstrate before drying is discontinued.
 12. The method of claim 1wherein up to 10% of the water in coated substrate remains in theadsorbent in the adsorbent structure after drying.
 13. The method ofclaim 1 additionally comprising exposing the sensitive adsorbent towater vapor before preparing the slurry, thereby slowly adding water tothe adsorbent before preparing the slurry.
 14. The method of claim 13wherein from about 20% to 30% by weight water is adsorbed by thesensitive adsorbent.
 15. The method of claim 13 wherein the water vaporis ambient humid air.
 16. The method of claim 13 wherein the exposingtakes place over a period of from about 4 to 7 days.
 17. The method ofclaim 1 wherein the substrate is a sheet and the adsorbent structure isa laminated sheet.